Anti-reflective silicon nitride film using in-situ deposition

ABSTRACT

An anti-reflective coating and method of forming the anti-reflective coating are described wherein the anti-reflective coating is part of a silicon nitride layer formed on a semiconductor integrated circuit substrate. The anti-reflective coating is formed under the photoresist layer for greater effectiveness but does not disrupt the process flow since the anti-reflective coating is part of the silicon nitride layer. A first silicon nitride layer is formed having an index of refraction of about 2.1. A second silicon nitride layer having an index of refraction of about 1.9 and a second thickness is formed on the first silicon nitride layer. A layer of photoresist is then formed on the second silicon nitride layer. The second thickness is chosen to be equal to the wavelength of the light used to expose the layer of photoresist divided by the quantity of 4 multiplied by 1.9. The second silicon nitride layer acts as an effective anti-reflective layer.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to an anti-reflective coating formed on thebottom of a photoresist layer and more particularly to the deposition ofa silicon nitride film having a low index of refraction, 1.9, over asilicon nitride film having a higher index of refraction, 2.1, to formthe anti-reflective coating.

(2) Description of the Related Art

U.S. Pat. No. 5,378,659 to Roman et al. describes the use of asilicon-rich silicon nitride layer, having an absorptive index greaterthan 0.25, as an anti-reflective coating for use in photolithographicprocessing.

U.S. Pat. No. 5,441,914 to Taft et al. describes using a thin siliconlayer between a patterned tungsten silicide layer and an overlyingpatterned silicon nitride anti-reflective layer to prevent delaminationof the anti-reflective layer.

U.S. Pat. No. 5,216,542 to Szczyrbowski et al. describes a five layersystem to provide an effective anti-reflective effect.

U.S. Pat. No. 5,126,289 to Ziger describes the use of an organicmaterial which is highly absorptive of deep ultra violet actinic lightto provide anti-reflection effects as well as surface planarization.

U.S. Pat. No. 5,449,639 to Wei et al. describes a method of metaletching using a disposable metal anti-reflective coating.

The anti-reflective coating described in this Patent Application uses asecond silicon nitride layer, having a low silicon to nitrogen ratio anda relatively low index of refraction, formed over a first siliconnitride layer, having a high silicon to nitrogen ratio and a relativelyhigh index of refraction, to provide an effective anti-reflectivecoating.

SUMMARY OF THE INVENTION

Patterns are typically formed in a layer of material, such as siliconnitride, on a semiconductor substrate by forming a photoresist layerover the layer of material, exposing the pattern in the photoresistlayer, developing the exposed photoresist layer, and using the developedphotoresist layer as a mask to form the pattern in the layer ofmaterial. The photoresist layer is exposed using light which has passedthrough a mask and is focussed on the photoresist layer. During theexposure of the photoresist layer light can enter the photoresist layerand set up multiple reflections within the photoresist layer. Themultiple reflections within the photoresist layer will causeconstructive and destructive interference at various points within thephotoresist layer which will degrade the pattern formed photoresistlayer and in the layer of material where the desired image is to beformed.

Anti-reflective coatings are often used to solve the problems caused bythe effect of standing waves in a layer of photoresist. Two types ofconventional anti-reflective coatings are shown in FIGS. 1A and 1B. FIG.1A shows a semiconductor substrate 10 with a layer of pad oxide 12formed thereon. A layer of silicon nitride 14 is formed on the layer ofpad oxide. In order to form a pattern in the layer of silicon nitride 14a layer of photoresist 16 is formed on the layer of silicon nitride 14.The photoresist is exposed using a light 30 which has passed through amask and focussed on the layer of photoresist 16. An layer ofanti-reflective material 18 is formed over the layer of photoresist toprevent multiple reflections within the layer of photoresist 16 fromsetting up standing waves and distorting the pattern formed in thephotoresist.

A second type of conventional anti-reflective coating is shown in FIG.1B. In this case the anti-reflective coating 18 is formed under thephotoresist layer 16 and over the layer of silicon nitride 14 to bepatterned. The anti-reflective coating shown in FIG. 1B with theanti-reflective coating under the photoresist layer is more effective inpreventing problems due to standing waves but this type ofanti-reflective coating increases the complexity of the process flow.

It is an objective of this Invention to provide a method of forming ananti-reflective coating over a silicon nitride layer and under aphotoresist layer which does not increase the complexity of the processflow and provides good anti-reflective characteristics.

It is another objective of this Invention to provide a method of forminga pattern in a layer of silicon nitride using an anti-reflective coatingunder the photoresist layer which does not increase the complexity ofthe process flow and provides good anti-reflective characteristics.

It is another objective of this Invention to provide an anti-reflectivecoating under a photoresist layer for use in forming a pattern in asilicon nitride layer.

These objectives are achieved by causing variations in the siliconnitride layer as it is being formed in order to use part of the siliconnitride layer as an in-situ anti-reflective coating. FIG. 2 shows adiagram of a light beam 30 illuminating a layer of first material 40having an index of refraction n₁, a layer of second material 42 havingan index of refraction n₂, and a layer of photoresist 44 having an indexof refraction n₃. The light has a wavelength λ₀. The layer of secondmaterial 42 will act as an effective anti-reflective coating preventingstanding waves in the photoresist layer 44 if the index of refraction ofthe second material, n₂, is equal to the square root of n₁ multiplied byn₃, and if the thickness, t, of the layer of second material 43 is equalto the wavelength, λ₀, divided by the quantity of the index ofrefraction of the second material, n₂, multiplied by four, λ₀ /(4n₂).

The index of refraction of the photoresist, n₃, is about 1.68 and theindex of refraction of silicon nitride can be varied from 1.9 to 2.1depending on the ration of silicon to nitrogen in the silicon nitridelayer. In this invention the layer of first material 40 is siliconnitride with a high silicon to nitrogen ratio and having an index ofrefraction of about 2.1. The layer of second material is silicon nitridewith a low silicon to nitrogen ratio having an index of refraction ofabout 1.9. The square root of 1.68 multiplied by 2.1 is 1.878 which isvery nearly equal to 1.9.

For an i line source having a wavelength of 3650 Angstroms the thicknessof the layer of second material, silicon nitride with an index ofrefraction of 1.9, is about 480 Angstroms which is 3650 Angstromsdivided by the quantity of 1.9 multiplied by 4. For an application usinga silicon nitride layer having a thickness of 1500 Angstroms thethickness of the layer of first material, silicon nitride with an indexof refraction of 2.1, is about 1020 Angstroms.

The objectives of the invention are achieved by forming a first siliconnitride layer with a high silicon to nitrogen ratio and an index ofrefraction of 2.1. A second silicon nitride layer with a low silicon tonitrogen ratio is formed on the first silicon nitride layer wherein thesecond silicon nitride layer has an index of refraction of 1.9 and athickness of 480 Angstroms. When a layer of photoresist is formed overthe second silicon nitride layer the second silicon nitride layer formsan effective anti-reflection layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross section view of a semiconductor integrated circuitwafer using an anti-reflective coating over the photoresist.

FIG. 1B shows a cross section view of a semiconductor integrated circuitwafer using an anti-reflective coating under the photoresist.

FIG. 2 shows a diagram of a light beam illuminating a layer ofphotoresist formed over a second layer of material which is formed overa first layer of material.

FIG. 3 shows a cross section view of a layer of pad oxide and a firstsilicon nitride layer formed on a semiconductor integrated circuitsubstrate wafer.

FIG. 4 shows a cross section view of the integrated circuit substrate ofFIG. 3 with a second silicon nitride layer formed on the first siliconnitride layer.

FIG. 5 shows a cross section view of the integrated circuit substrate ofFIG. 4 with a photoresist layer formed on the second silicon nitridelayer and a light beam illuminating the photoresist layer.

FIG. 6 shows a block diagram of an exposure apparatus used to expose thephotoresist layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer now to FIGS. 3, 4, and 5, there is shown the preferred embodimentof the anti-reflective coating of this invention. As shown in FIG. 3, alayer of pad oxide 12 is formed on a semiconductor integrated circuitsubstrate 10. Next, a first silicon nitride layer 20 having a firstthickness 21 is formed on the layer of pad oxide using a method such aslow pressure chemical vapor deposition, plasma enhanced chemical vapordeposition, or the like. The deposition of the first silicon nitridelayer is, adjusted to give a high silicon to nitrogen ratio, greaterthan 0.75, resulting in an index of refraction of between about 2.08 and2.12 and a first thickness 21 of between about 1410 and 1590 Angstroms.

Next, as shown in FIG. 4, a second silicon nitride layer 22 having asecond thickness 23 is formed on the first silicon nitride layer 20using a method such as low pressure chemical vapor deposition, plasmaenhanced chemical vapor deposition, or the like. The deposition of thesecond silicon nitride layer is adjusted to give a low silicon tonitrogen ratio, less than 0.75, resulting in an index of refraction ofbetween about 1.88 and 1.92 and a second thickness 23 of between about450 and 510 Angstroms.

Next, as shown in FIG. 5, a layer of photoresist 26, having an index ofrefraction of about 1.68, is formed on the second silicon nitride layer22. The second silicon nitride layer 22 will form an effectiveanti-reflective coating when the photoresist is illuminated with lighthaving a wavelength of about 3650 Angstroms. For light having adifferent wavelength a different second thickness 23 for the secondlayer of silicon nitride 22 must be used. The second thickness 23 of thesecond silicon nitride layer 22 is chosen to be equal to the wavelengthof the light used divided by the quantity of 4 multiplied by the indexof refraction of the second silicon nitride layer, in this example 1.9.

As shown in FIG. 5, the photoresist layer 26 is then illuminated bylight 30 which has passed through a mask and is focussed on thephotoresist layer 26. In this example the light is from an i line sourcehaving a wavelength of 3650 Angstroms. FIG. 6 shows a block diagram ofan exposure apparatus, such as a five times reduction stepper, used toexpose the photoresist layer 26. A light beam 42 is produced by a lightsource 40, in this example an i line source having a wavelength of 3650Angstroms, and directed to a mask and mask holder apparatus 44. A lightbeam 30 emerges from the mask and mask holder apparatus 44 and isfocussed on the photoresist layer 26. The semiconductor integratedcircuit substrate 10, pad oxide layer 12, first silicon nitride layer20, second silicon nitride layer 22, and photoresist layer 26 are aspreviously described.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of forming an anti-reflective coating,comprising the steps of:providing a source of light having a firstwavelength; providing an integrated circuit substrate; forming a padoxide layer on said integrated circuit substrate; depositing a firstsilicon nitride layer having a first thickness and a first index ofrefraction on said pad oxide layer wherein the deposition of said firstsilicon nitride layer is adjusted so that said first index of refractionis between 2.08 and 2.12; depositing a second silicon nitride layerhaving a second thickness and a second index of refraction on said firstsilicon nitride layer wherein the deposition of said second siliconnitride layer is adjusted so that said second index of refraction isbetween 1.88 and 1.92 and said second thickness is one fourth of saidfirst wavelength divided by said second index of refraction; and forminga photoresist layer on said second silicon nitride layer wherein saidphotoresist layer has an index of refraction of between 1.66 and 1.70and said photoresist layer will be exposed using said source of lighthaving said first wavelength.
 2. The method of claim 1 wherein saidfirst silicon nitride layer has a ratio of silicon to nitrogen ofgreater than 0.75.
 3. The method of claim 1 wherein said second siliconnitride layer has a ratio of silicon to nitrogen of less than 0.75. 4.The method of claim 1 wherein said first wavelength is 3650 Angstromsand said second thickness is between 450 and 510 Angstroms.
 5. Themethod of claim 1 wherein the sum of said first thickness and saidsecond thickness is between 1410 and 1590 Angstroms.
 6. A method offorming a pattern in a photoresist layer using an anti-reflectivecoating, comprising the steps of:providing a source of light having afirst wavelength; providing an integrated circuit substrate; forming apad oxide layer on said integrated circuit substrate; depositing a firstsilicon nitride layer having a first thickness and a first index ofrefraction on said pad oxide layer wherein the deposition of said firstsilicon nitride layer is adjusted so that said first index of refractionis between 2.08 and 2.12; depositing a second silicon nitride layerhaving a second thickness and a second index of refraction on said firstsilicon nitride layer wherein the deposition of said second siliconnitride layer is adjusted so that said second index of refraction isbetween 1.88 and 1.92 and said second thickness is one fourth of saidfirst wavelength divided by said second index of refraction; forming aphotoresist layer on said second silicon nitride layer wherein saidphotoresist layer has an index of refraction of between 1.66 and 1.70;and exposing a pattern in said photoresist layer using said source oflight having said first wavelength.
 7. The method of claim 6 whereinsaid first silicon nitride layer has a ratio of silicon to nitrogen ofgreater than 0.75.
 8. The method of claim 6 wherein said second siliconnitride layer has a ratio of silicon to nitrogen of less than 0.75. 9.The method of claim 6 wherein said first wavelength is 3650 Angstromsand said second thickness is between 450 and 510 Angstroms.
 10. Themethod of claim 6 wherein the sum of said first thickness and saidsecond thickness is between 1410 and 1590 Angstroms.